1. Field of the Invention
The present invention relates to a semiconductor laser device and, more particularly, to a ridge waveguide type semiconductor laser device.
2. Description of the Background Art
There are semiconductor laser devices having various structures. In particular, it is noted that some semiconductor laser devices have a channel waveguide having a refractive index structure for confining light in an active layer in a horizontal direction parallel to the layer. For example, a ridge waveguide type semiconductor laser device is a channel waveguide type semiconductor laser device. The ridge waveguide type semiconductor laser device has a ridge portion (that is, projection portion) projected from the peripheral cladding layer such that current is injected into the ridge portion for laser oscillation.
The ridge waveguide type semiconductor laser device will be described below with reference to FIGS. 1 to 3. As shown in FIGS. 1 and 2, the semiconductor laser device 60 has a stacked structure including a first electrode 56a, an n-type GaAs substrate 51, an n-type AlGaInP first cladding layer 52, an active layer 53, a p-type AlGaInP second cladding layer 54, an insulating layer 55, a p-type GaAs contact layer 57, and a second electrode 56b. The active layer 53 has multiple quantum wells (hereinafter referred to as MQW) structure. The second cladding layer 54 has a projection portion 58 that projects. The insulating layer 55 covers the portion except for the top thereof, and it is made of a silicon nitride (SiN) layer having a layer thickness of 100 nm. The second electrode 56b is electrically connected to the top of the projection portion 58 of the second cladding layer 54 through the contact layer 57. The first and second electrodes 56a and 56b are made of a metal such as AuGe/Ni/Au and Ti/Au, respectively. In addition, a window region 59 is formed by diffusion of zinc atoms within the active layer 53, adjacent to a facet of the semiconductor laser device 60. A laser beam 62 is output to the outside through the end-face window region 59.
FIG. 3 shows optical output dependency of a far field pattern (hereinafter referred to as FFPx) in a direction parallel to a p-n junction plane of the conventional ridge waveguide type semiconductor laser device. As shown in FIG. 3, a half width of FFPx is increased as the optical output is increased. There is a relatively large difference of about 2xc2x0 between half widths of FFPx at an output of 5 mW and at an output of 50 mW (hereinafter referred to as xcex94FFPx). It is noted that the half width of FFPx at 50 mW is measured but FFPx at 50 mW isn""t shown in the figure. When the optical output is changed, if xcex94FFPx is large, optical design in an application device such as a DVD-R drive becomes disadvantageously complicated.
It is, therefore, an object of the present invention to provide a semiconductor laser device in which xcex94FFPx is not changed largely even when optical output thereof is changed.
In accordance with one aspect of the present invention, there is a semiconductor laser device including a stacked structure. The stacked structure includes a first electrode, a substrate of first conduction type layered on the first electrode, a first cladding layer of the first conduction type, an active layer, a second cladding layer of second conduction type opposite to the first one, an insulating layer, and a second electrode. The second cladding layer is deposited on the active layer. The second cladding layer includes at least first and second portion having thickness different from each other. The first portion is thicker than the second portion. The first portion is referred to as projection portion. The insulating layer is deposited on the second cladding layer except for the first portion. The second electrode electrically is connected to the first portion of the second cladding layer, and is deposited on the insulating layer. In the insulating layer, a product of a reciprocal of a layer thickness and a heat conductivity of the insulating layer is smaller than 4xc3x97108 W/(m2K).